Call it the case of the missing "greenhouse gas." For years, scientists have been trying to figure out where carbon dioxide goes once humans generate it. Significant amounts billow into the atmosphere. But each year, atmospheric CO2 concentrations have been rising only half as fast as humans supply the gas.

The hiding place, it turns out, is the world's oceans. And the implications for marine life are troubling, researchers say. If industrial CO2 emissions continue to increase at their current rate, by the end of the century the surface waters of the world's oceans are likely to become more acidic. Though the change appears subtle, it could threaten key organisms at the base of the marine food chain and further endanger shallow-water reefs, which represent some of the most biologically productive ecosystems on the planet. The absorption of this extra carbon dioxide would induce changes in ocean chemistry not seen for at least 20 million years, some researchers say.

"A lot has been said about carbon dioxide in the atmosphere" and its impact on climate, says John Raven, a marine biologist at the University of Dundee in Scotland. But geophysical chemistry also "has pretty firmly established that when CO2 in the atmosphere dissolves into the ocean, the surface ocean will become more acidic. That is something that is happening."

The oceans have been viewed as a potential brake on global warming through natural and engineered approaches to storing carbon- dioxide there. Ironically, it now seems that increased ocean uptake of CO2 "is not a good thing overall," he says.

One sign of growing concern over the issue comes from Britain, where the Royal Society - Britain's equivalent to the US National Academy of Sciences - late last month commissioned a six-month study to see what current research has to say about the ocean's carbon uptake and its effect on marine life. Prime Minister Tony Blair, who is slated to become president of the Group of Eight industrial countries next year, is said to have climate change high on his list of priorities for the next G-8 meeting. The Royal Academy study is expected to play a key role in those discussions.

The Royal Academy's announcement came a month after research results appeared in the US journal Science that solved the mystery of the missing CO2 and considered its biochemical consequences.

A research team, led by marine chemist Christopher Sabine, took on the herculean task of compiling a global picture of the oceans' CO2 uptake, based on measurements from some 70,000 samples of seawater. The samples were collected worldwide during two large oceanographic projects in the late 1980s and 1990s aimed at measuring ocean circulation and the movement of carbon through the system.

"We've known for years that the oceans take up a significant amount of carbon dioxide," says Dr. Sabine, with the Pacific Marine Environmental Laboratory in Seattle, part of the National Oceanic and Atmospheric Administration (NOAA). "But we haven't been able to quantify it based on direct measurements until now."

From 1800 to 1994, the team estimates, the oceans soaked up 48 percent of the carbon emitted from human activities, such as burning wood, coal, oil, or gas. Thus, the oceans are currently storing about a third of their long-term potential, the team concluded.

The real surprise, however, came from the impact the results had on the overall picture of the globe's carbon cycle,

When the team added the carbon stored in the oceans to the carbon stored in the atmosphere, the total exceeded emissions from human activities alone. After carefully reviewing their data and calculations, they concluded that the "extra" CO2 came from changes in land use, such as deforestation. This suggests that during the same period, the planet's terrestrial bio-sphere became a net source of, rather than a sink for, carbon dioxide.

Marine biologists, meanwhile, worry about what happens to that carbon once the oceans take it up.

When carbon dioxide mixes with seawater, it forms a weak carbonic acid. Over millions of years, erosion has supplied the oceans with vast amounts of dissolved calcium from weathered rock on the continents. This provides a natural buffer against the acid, creating chemical conditions to which some key forms of marine life are finely tuned.

Over the past five years, however, evidence has been mounting that rising CO2 levels could pose major challenges to these life forms by altering this balance.

By some measures, rising CO2 levels during the industrial age already have increased the oceans' acidity by roughly 0.1 pH units. By the end of this century, the reduction could reach 0.4 units. That may not sound like much, but researchers point out that each whole-number shift in pH represents a 10-fold change.

Thus, to some marine organisms, a pH shift of 0.4 toward the acid end of the scale could lead to dramatic changes.

By the middle of the next century, for example, coral reefs in shallow waters could lose up to 30 percent of the calcium carbonate they need to build their structures, calculates an international research group led by Joan Kleypas, a marine biologist at the National Center for Atmospheric Research in Boulder, Colo. That could lead to stunted growth or other effects, which could make them more vulnerable to erosion or storm damage.

Also, increased CO2 levels caused key plankton species to create badly formed or incomplete calcium carbonate shells, according to a team led by Ulf Riebesell with the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. In lab experiments the ratio of shell to the rest of the organism dropped by as much as 52 percent.

"We've only just scratched the surface on this issue," says Victoria Fabry, a biologist at the California State University at San Marcos who specializes in plankton ecology. Preliminary results from her own studies show that when water is "undersaturated" with a certain form of calcium carbonate, the shells of tiny plankton-like creatures start dissolving within 48 hours.

Ironically, chemistry that threatens the organisms can help the atmosphere. As shells and other calcium-carbonate clothing dissolve, it returns minerals to the seawater that can help the oceans soak up more CO2. Research by a team led by Richard Feely, also with NOAA's lab in Seattle, notes up to 60 percent of the calcium carbonate formed each year dissolves in the upper 2,000 meters of the ocean, making it readily available as a buffer. But it also could be reducing the amount of carbon that falls to the deep ocean, where it is cached for centuries, or gets buried in sediments.

The net effect of these processes remains unclear both for the atmosphere and marine ecosystems - leading researchers to undertake more detailed studies. Next year researchers will head to sea to gather more information on the amount of carbon the ocean is taking in to see what changes may be taking place.

"The world ocean is one of the great commons of the human race," says Dr. Raven. "We're all stakeholders, and what we're doing influences it."